1. Field of the Invention
The present invention generally relates to spread spectrum systems and, more particularly, to a hybrid rake/equalizer receiver for a spread spectrum system.
2. Background of the Invention
In wireless communications systems, signals propagate over many paths from the transmitter to the receiver. Delay diversity occurs in radio systems when the difference in signal arrival time between the shortest path and longest path exceeds the symbol duration. Delay diversity leads to inter-symbol interference (ISI) in classical communications systems, which degrades performance. Spread spectrum systems take advantage of delay diversity by learning the channel, then despreading and combining the energy from resolvable paths. Codes are designed to minimize the effect of inter-chip interference (ICI) caused by delay diversity. However, when multiple downlink data streams are multiplexed and transmitted via orthogonal codes, delay diversity destroys orthogonality, creating severe multi-access interference (MAI). The channel is time varying, which requires an adaptive learning process. Often, the delay spread in spread spectrum systems is very long compared to the chip duration, e.g., up to 100 chips. Furthermore, the delay spread is sparse, i.e., few energy regions, separated by nulls, characterize the delay spread.
Typical mobile units in Code Division Multiple Access (CDMA) systems employ a rake receiver, i.e., a channel matched filter, to take advantage of sparse delay diversity. A reduced complexity rake receiver combines energy from a few strongest paths, without regard to reducing MAI. Recently, a chip-level equalizer has been proposed to restore orthogonality, thereby reducing MAI in the bit estimates. Unfortunately, adaptively equalizing the entire channel delay spread with a single finite impulse response (FIR) equalizer requires complexity greater than the resources of current mobile units.
Accordingly, it would be desirable and highly advantageous to have a hybrid rake/equalizer receiver for a spread spectrum system that overcomes the above-described problems of the prior art.